1. Field of the Invention
The present invention relates to an apparatus for straightening curls in a sheet made of a plastic film, paper, a metallic foil or the like which is wound off from a winding roll.
2. Description Of The Prior Art
For example, in an apparatus for cutting and treating a sheet, the sheet wound off from a winding roll is cut into given lengths by a cutting machine. At this time, if the sheet wound off from the winding roll is cut in that state, curls come into the open in the cut sheets due to a wound form. Such curls create problems such as jamming which may occur in a line for cutting and treating the sheets and in the sheet inlet portion of a printing machine or of various other machines when the sheets are thereafter introduced therein.
An apparatus for straightening curls in a sheet has previously been proposed in which a wrap angle is provided for a sheet wound off from a winding roll by applying a decurler bar to the sheet between two backup rolls, as described in Japanese Patent Publication No. 48427/1985.
The degree to which curls are straightened by the above-described apparatus for straightening sheet curls depends upon the wrap angle of the sheet, and the greater the wrap angle is, the stronger the pressure with which the sheet is wiped with the decurler bar and the greater the degree to which curls are straightened.
On the other hand, when a sheet is wound off from a winding roll, the extent to which curls are present in the sheet increases as the diameter of the winding roll decreases while winding-off progresses.
The use of the above-described apparatus for straightening sheet curls therefore requires monitoring of the change in the diameter of the winding roll and the provision of a wrap angle adjusting device which is capable of adjusting the relative position between the backup rolls and the decurler bar in such a manner that the wrap angle of the sheet is increased as the diameter of the winding roll decreases.
For example, such a wrap angle adjusting device is so configured that the decurler bar is displaced from a position corresponding to a large parent roll (the position where a low degree of pushing force is applied inwardly) to a position corresponding to a small parent roll (the position where a high degree of pushing force is applied inwardly ) in the state wherein the backup rolls are set at standard positions with the sheet pushed in.
In the apparatus for straightening sheet curls described in Japanese Patent Publication No. 48427/1985, an optimum degree of driving of the decurler bar for the purpose of straightening curls in correspondence with the change in diameter of the winding roll is expressed in a digital pattern using figures in order to automatically adjust the degree of straightening of the curls in the sheet in correspondence with the change in diameter of the winding roll. The figures are separately formed for respective kinds of sheet and the respective component items of the relevant straightening apparatus, each of the figures being formed by collecting in advance data with respect to the optimum degree of driving of the decurler bar from experiments which are performed in such a manner that the diameter of the winding roll is changed within a given range.
There has been an increasing demand in recent years for a hgh level of operational stability and a high level of productivity to be available in, for example, an apparatus for cutting and treating a sheet which is provided with an apparatus for straightening sheet curls, Improvements in the operating properties of such apparatus for cutting and treating sheets have not, however, been realized because the apparatus for straightening sheet curls represents a bottleneck. This is because of the problems described below which are not easily resolved, which are inherent to conventional control methods for controlling the driving of a decurler bar in the manner of a digital pattern, and which inhibit attempts to automate the straightening of curls from a simple arrangement in a rapid and highly precise manner.
(1) In order to increase the precision with which curls are straightened, it is necessary to set the driving of the decurler bar at an appropriate amount at each time the diameter of the winding roll slightly changes, resulting in the use of a large volume of data. The collection of data therefore requires a great number of processes.
2. Such data is previously determined for each kind of sheet and the component items of the straightening apparatus and are stored in the data storage memory so that data conforming to the relevant operation of straightening may be transmitted to the controller of the wrap angle adjusting device from the data storage memory and then used. During this operation, since, as described above, the volume of data is large, it is necessary to ensure that both the data storage memory and the controller have a space with a large memory capacity, and the further transmission of data from the data storage memory to the controller takes a great deal of time.
3. Since the operation of controlling the decurler bar using the wrap angle adjusting device is intermittently (stepwisely) performed in correspondence with the change in diameter of the winding roll, an optimum wrap angle cannot be set for the diameter of the winding roll each moment as it changes.
It is an object of the present invention to realize the automatization of straightening of curls with a simple arrangement and in a rapid and highly precise manner.